perf: add defer to scripts and lazy loading to images

Add defer attribute to 145 <script> tags across 103 template files
(PERF-067) and loading="lazy" to 22 <img> tags across 13 template
files (PERF-058). Both improve page load performance.

Validator totals: 0 errors, 2 warnings, 1360 info (down from 1527).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/templates/shared/macros/datepicker.html

@@ -7,7 +7,7 @@
     Add Flatpickr CDN to your base template:
     <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/flatpickr/dist/flatpickr.min.css">
     <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/flatpickr/dist/themes/dark.css">
-    <script src="https://cdn.jsdelivr.net/npm/flatpickr"></script>
+    <script defer src="https://cdn.jsdelivr.net/npm/flatpickr"></script>
 
   Usage:
     {% from 'shared/macros/datepicker.html' import datepicker, daterange_picker, datetime_picker %}